1. Field of the Invention
The present invention relates generally to photographic cassettes, and more particularly to an improved photographic cassette, such as an x-ray film sheet cassette.
2. Description of the Prior Art
U.S. Pat. Nos. 3,784,835, 3,870,889 and 4,444,484 describe x-ray film sheet cassettes of the type suitable for day-light loading and unloading in cooperation with film supplying and film processing apparatus. Such cassettes typically have a rectangular solid configuration defined by six orthogonally joined walls which enclose a chamber for accommodating an x-ray film sheet. In one of those walls is a light-shielded slot through which a film sheet can pass into and out of the chamber. Within the chamber, in alignment with the slot, are a pair of opposing planar screen surfaces relatively moveable toward and away from each other to prevent and permit, respectively, movement of the film sheet therebetween. One of those surfaces is secured to a wall adjacent thereto, and the opposed surface is forced by a spring or other means toward the other to clamp a film sheet therebetween. Means are provided for urging the two surfaces apart in response to placing the cassette into operative relationship with suitably configured film supplying or film processing apparatus, thereby allowing a film sheet to slide between those surfaces when the cassette is positioned for loading or unloading. When so positioned, the cassette is intended to be loaded or unloaded by the simple expedient of letting a film sheet drop into or out of the chamber by force of gravity.
A problem with the aforementioned cassettes, particularly of the type shown in U.S. Pat. No. 4,444,484, is that when an external force is applied to the weight-bearing cassette front cover wall by the weight of a portion of a patient's anatomy being x-rayed, such as an elbow, for example, the cover and screen secured thereto are boxed resulting in good line film-to screen contact at the center of the longitudinal length of the cassette, but a progressive loss in film-to-screen contact upon outward progression to the ends of the cassette. This loss in film-to-screen contact is due to the screen ends secured to the bowed cover withdrawing from the film as it is pressed toward the boxed cover by the pressure applying means.
The aforementioned problem of loss of film-to-screen contact at the screen ends is solved by a cassette construction of the type disclosed in U.S. patent application Ser. No. 879,245 by the same assignee of the present invention. The cassette disclosed in U.S. Ser. No. 879,245 is constructed so that the screen adjacent the bowed front cover wall facing the x-ray machine is free-floating, at least those portions thereof extending progressively outwardly from the center of the cassette, when the cassette is in a film-loaded position for exposure. In U.S. Ser. No. 879,245, the specific mechanism for separating the pressure and floating plates to allow a film sheet to freely enter or exit the passageway therebetween comprises metal bell crank levers, each comprising two longitudinally projecting arms which are joined to form an obtuse angle therebetween. Upon pivotable movement of the bell crank levers, one of the arms thereof engages and lifts a tab on the pressure plate, and the other arms each have a lug thereon that engages a complementary ear on the floating plate for separating the pressure and floating plates.
The aforementioned problem is better solved by the present invention by a mechanism that is of simple design and construction, thoroughly reliable and efficient in operation, more durable, more dirt free during usage, constructed of fewer parts, and more economical to manufacture.